BERLIN (Reuters) - Germany's center-left chancellor candidate Martin Schulz has criticized U.S. President Donald Trump's policies as "un-American" and warned against lifting sanctions imposed against Russia over its role in the Ukraine crisis.

In one of the sharpest remarks yet by a senior German politician since the new American leader took office, Schulz told the Funke media group in an interview published on Wednesday that Europe had to stand up to defend liberal values.

"What Trump is doing is un-American," Schulz said, adding that the United States like no other country in the world stood for enlightenment, democracy and freedom.

"If Trump is now driving a wrecking ball through this set of values, then I will tell him as chancellor: That's not the policy of Germany and Europe," Schulz added.

Germany's Social Democrats last week nominated former European Parliament president Schulz to run against Chancellor Angela Merkel in a federal election in September, and the surprise move has boosted popular support for the centre-left party.

Merkel said on Monday the global fight against terrorism does not warrant putting groups of people under suspicion, adding Trump's order to restrict people from seven Muslim-majority countries from entering the United States violates the spirit of international cooperation.

Turning to Russian President Vladimir Putin and the Ukraine conflict, Schulz said that sanctions imposed against Russia could only be lifted after both sides had implemented the so-called Minsk peace plan.

"As long as the Minsk peace agreement is not fully implemented, the sanctions cannot be lifted. We must tell Putin very clearly that Russia is obliged to respect and defend international law," Schulz said.

Schulz' comments bring more clarity about his stance on Russia after some leading Social Democrats have voiced support for a partial lifting of sanctions as long as Russia is implementing some aspects of the plan.

Merkel, one of the architects of the peace deal, has repeatedly said that sanctions against Russia can only be lifted once the Minsk agreement has been fully implemented.